This invention relates to a class of glass compositions which spontaneously undergo opalization upon melting and forming and which are capable of being strengthened by a conventional surface ion exchange treatment. Such glasses have potential application for use as break-resistant tableware, containers for products such as cosmetics and pharmaceuticals, decorative panels and tile, architectural members, electrical insulators, and many other products.
The terms "opal," "opalized," or "opalescence" as used herein are intended to refer to the propensity of certain glasses to scatter a substantial portion of the light passing therethrough so as to render visibility through the glass essentially nil. Opalescence may vary by degrees from a cloudy iridescence to a nearly opaque white, and is dependent upon the thickness of a given piece. Observations of the degree of opalescence herein are based on samples (plates and/or rods) having thicknesses commensurate with at least some of the above-noted proposed applications--about 1/8 inch (3 millimeters) to about 1/4 inch (6 millimeters).
Fluorine has long been used as an opalizing agent in glass, but its use has been discouraged by air pollution problems associated therewith. Proposed substitutes for fluorine have been disclosed, for example, in the following:
U.S. Pat. No. 2,559,805-- Stookey
U.S. Pat. No. 3,498,801 -- Keul
U.S. Pat. No. 3,709,705 -- Hagedorn
U.S. Pat. No. 3,647,490 -- Pirooz
U.S. Pat. No. 3,661,601 -- Dambaugh et al.
W. German Application No. P 23 13 074.7
The Stookey patent mentions calcium phosphate and barium phosphate as opacifiers, but because of various drawbacks, teaches the use of only a specific barium phosphate composition. In the Keul patent, P.sub.2 O.sub.5 and CaO serve as the opalizers. B.sub.2 O.sub.3 is the primary opalizing agent in the Hagedorn and Pirooz patents. Dumbaugh et al. disclose the combination of F, B.sub.2 O.sub.3, and CaO to be an active opalizer. The West German application employs B.sub.2 O.sub.3, K.sub.2 O and P.sub.2 O.sub.5 as the opalizing ingredients. However, none of these prior opal glasses is intended for chemical strengthening.
Chemical strengthening (or "chemical tempering") of glass involves an exchange of ions near the surface of the glass article with ions from an external source, typically a molten inorganic salt bath, the object being the generation of a zone near the surface of the glass which is in a state of compression relative to the interior portions of the glass. There are two types of ion exchange strengthening which differ substantially in theory and operation. The first type of ion exchange treatment is carried out above the strain point of the glass and has as its object the alteration of the glass composition at the surface so as to lower the thermal coefficient of expansion in the surface layer. As the glass is cooled, a compressive stress develops at the surface due to the expansion differential. This approach was taught by Hood and Stookey in U.S. Pat. No. 2,779,136. The second type of ion exchange strengthening is characterized by treatment below the strain point of the glass, wherein surface compression is generated by substituting large ions from an external source (e.g., a molten salt bath) for smaller ions in the glass. Typically, the substitution is of sodium or potassium for lithium in the glass, or of potassium for sodium in the glass. The below-the-strain-point technique was first taught by Weber in U.S. Pat. No. 3,218,220.
Of the two types of ion exchange strengthening, the second (below the strain point) type is preferred for large-scale commercial use. This is because maintaining the glass below its strain temperature avoids causing undesirable distortion defects in the glass. Furthermore, since it is costly to include lithium in a glass as a batch ingredient, and because greater strengthening can generally be achieved, it is desirable that sodium, rather than lithium, be the ion in the glass which is replaced. In that case, the larger ion which enters the glass is most advantageously potassium. Hence, this invention is directed specifically to the improvement of ion exchange strengthening processes which involve replacing sodium with potassium below the strain point of the glass.
Conventional soda-lime-silica flat glass compositions can be strengthened by ion exchange, but the greater length of time required to produce a significant compression layer depth is incompatible with many high volume commercial operations. For this reason, special glass compositions have been developed which have greatly enhanced ion exchange properties, chief amoung which are the Al.sub.2 O.sub.3 and/or ZrO.sub.2 containing glasses disclosed by Mochel in U.S. Pat. Nos. 3,485,702; 3,752,729; and 3,790,430. Variations of these alumina or zirconia containing glasses may be seen in many U.S. Patents including the following:
U.S. Pat. No. 3,357,876 -- Rinehart
U.S. Pat. No. 3,433,611 -- Saunders et al.
U.S. Pat. No. 3,481,726 -- Fischer et al.
U.S. Pat. No. 3,485,647 -- Harrington
U.S. Pat. No. 3,498,773 -- Grubb et al.
U.S. Pat. No. 3,778,335 -- Boyd
U.S. Pat. No. 3,844,754 -- Grubb et al.
U.S. Pat. No. 3,772,135 -- Hara et al.
None of the above-cited patents discloses an ion exchange glass which is opalized, although U.S. Pat. No. 3,485,647 to Harrington mentions a "slight opal" appearance in two samples of his ion exchange glasses. Not only is the opalescence of the Harrington glasses only slight, but the glasses are intended for use in the less desirable type of ion exchange process where lithium ions replace sodium ions above the strain point of the glass.
A different type of strengthened opal glasses are disclosed in U.S. Pat. No. 3,907,577 to Kiefer et al., but they are not strengthened by ion exchange with an external source of ions as in the present invention. Rather, Kiefer thermally induces crystal formation at the glass surface by a migration of ions within the glass, and the crystal layer produced creates a compression layer at the surface. Kiefer's glasses require the inclusion of lithium for crystal formation, which renders them less desirable for ion exchange treatment for the economic and strength considerations previously mentioned. A lithium-containing glass is furthermore not suitable for exchanging sodium ions in the glass with a potassium exchange bath because the lithium ions would tend to accumulate in the bath, thereby reducing its ion exchanging effectiveness. Although the Kiefer patent does mention the effect of P.sub.2 O.sub.5, CaO, and BaO on the opacity of his glasses, he does not teach the class of ion exchange glass to which the present invention is directed.
While the specially adapted ion exchange glass compositions of the prior art greatly reduce the amount of time required for ion exchange treatment compared to conventional soda-lime-silica glass, their commercial use remains limited to low volume specialty items because treatment times are still impractically long for many applications. Moreover, many of the prior art compositions have melting temperatures considerably higher than soda-lime-silica glass and thus are not readily adapted for use in existing melting and forming facilities. Thus, it would be highly desirable to have glass compositions available which are not only opalized, but could also be more rapidly strengthened by exchange treatment with potassium and have melting temperatures more in line with ordinary soda-lime-silica glass. Other factors such as transparency, chemical durability, and the cost of raw materials also must be taken into consideration.
Substantial progress toward the above-noted goals for clear glasses was attained by the glass compositions disclosed in parent application Ser. No. 605,108, the disclosure of which is hereby incorporated by reference. The present invention represents an even greater improvement over those glasses, particularly in regard to the speed with which a deep compression layer can be created in the glasses, as well as providing opalescence.